The availability of low-cost flash memory has been a major facilitator in the widespread proliferation of portable electronic devices such as smart phones, personal digital assistants, tablet and notebook computers, digital cameras, digital audio players, etc. It has also allowed the production of low-cost, flash-based solid-state drives (SSDs) which provide long-term persistent storage, similar to traditional hard disk drives (HDDs) but without the need for any moving parts. Flash memory is non-volatile, meaning that it retains its stored information even when not powered. It is also electrically erasable and reprogrammable, light-weight and durable, and requires no moving parts. All of these attributes lend well for use in portable electronic devices.
To satisfy demand for higher capacity flash memory while keeping manufacturing costs low, flash memory manufacturers have resorted to process scaling techniques in which the memory cells that make up flash memory—known as “floating gate transistors”—are fabricated with smaller dimensions. By scaling down (i.e., “shrinking”) the dimensions of the individual floating gate transistors, higher capacity flash memory can be produced. Over the years, process scaling has proved to be remarkably successful, reducing the minimum feature size of floating gate transistors from around 1 micron (1,000 nanometers) in the early 1990s to around 25 nanometers today. However, the ability to scale down further is impeded by diffraction limits of the photolithography processes used in fabricating the floating gate transistors and by short channel effects and memory retention problems that arise when floating gate transistors are scaled down to nanometer dimensions.
Various alternative non-volatile memory technologies have been proposed to replace floating gate transistor memory cells, including phase-change memory cells, in which thermal processes are used to control amorphous and crystalline phase transitions in a chalcogenide; magnetoresistive memory cells, in which magnetizations of ferromagnetic films are used to inhibit or allow electron tunneling through intermediate insulating films; and resistive change memory cells, in which electric fields are used to control ionic transport and electrochemical redox reactions in transition metal oxides. Some of these alternative non-volatile memory technologies have shown great promise. However, various challenges to integrating the memory cells in high-density memory arrays remain. To compete with existing flash memory, memory cell densities of non-volatile memory technologies must rival and preferably exceed state-of-the-art flash memory cell densities.